In the rapid transfer and positioning of a shuttle or carriage which is supported for reciprocating linear movement, it is known to use an endless or open-ended flat flexible timing belt having longitudinally spaced parallel teeth. The toothed belt extends around a drive sprocket having mating teeth and connected to a drive shaft which is driven by a reversible drive. The toothed timing belt also extends around an idler or tail sprocket having mating teeth and rotatably supported by a tail shaft which extends parallel to the drive shaft. The spacing between the drive shaft and the tail shaft determine the extent of linear travel of the carriage which is commonly supported by parallel spaced tracks.
When the shuttle or carriage positioning and transfer system or apparatus is used for rapidly moving a substantial load, for example, when used with automotive assembly machinery for moving components of automotive vehicles, the load carrying capacity of the timing belt usually determines the load moving capacity of the shuttle or carriage. If two timing belts are used to move a shuttle or carriage, the variation in the belts due to production tolerances, results in one of the belts carrying a greater portion of the load than the other belt. Thus if the one belt is operated at its full rated capacity, the second belt does not significantly increase the load carrying capacity of the belt system, and primarily offers only a safety factor.